The goal of this project is to compare the effectiveness of two methods (partner contact tracing [PCT] versus venue-based [VB] recruitment) for identifying previously undiagnosed HIV+ cases among men who have sex with men (MSM) in Tijuana, Mexico. We will also use self-reported sexual networks, high-risk venue-based networks, and viral genetic networks (inferred by analysis of viral pol sequences) to identify missing connections (e.g., anonymous partners) in sexual transmission networks. In our MSM pilot data, HIV prevalence was 20.2%, and 88% of those were unaware of their HIV+ serostatus. Identifying previously undiagnosed HIV+ persons and linking and retaining them in care are critical steps in the treatment cascade, particularly among MSM in Mexico, who are disproportionately affected by HIV. No published studies have examined case finding methods in combination with genotyping to infer sexual transmission networks. Three innovative aims will be addressed: Aim 1. Compare the effectiveness of PCT versus screening attendees of high-risk venues (e.g., sex clubs) as methods for finding previously undiagnosed HIV+ cases among MSM in Tijuana. Aim 2. Determine the extent to which sexual contact networks generated through PCT versus VB sexual networks (generated through data on MSMs' affiliations in high-risk venues) map onto viral genetic networks at baseline and one-year follow-up. Aim 3. Examine PCT versus VB case finding methods and their relationships to (a) linkage to and retention in HIV care and their psychosocial and environmental determinants; and (b) stability of sexual contact networks assessed over one-year follow-up. We will compare the productivity (i.e., ratio of undiagnosed cases to number of men screened) of these two case finding methods (n=200/group) in identifying previously undiagnosed MSM (primary outcome). By comparing these forms of network creation to HIV pol sequence data, we aim to identify where gaps exist in the chain(s) of transmission and thereby increase our understanding of the structure of local transmission networks. Longitudinal data will allow us to examine stability of sexual networks, as well as barriers and facilitators of linkage to and retention in care. This is the first study t: 1) determine the effectiveness of case finding methods to identify previously undiagnosed HIV+ cases in a high-risk, understudied population, using state-of-the-art network analyses; 2) use multiple forms of network creation to identify missing connections in sexual contact networks; and 3) identify factors associated with linkage and retention in care among HIV+ MSM in a lower- to middle-income country where this information is severely lacking.